1. Field of the Invention
The present invention relates to a method for producing a zinc coated steel sheet having excellent corrosion resistance suitable for automobile sheets.
2. Description of the Related Arts
Steel sheets of an automobile body are mainly rust-resistant steel sheets from the viewpoint of reliability to sustaining rust preventive performance for a long period of service. Among various kinds of the rust-resistant steel sheets, a zinc coated steel sheet and an alloyed zinc coated steel sheet are most widely used. Apart from the zinc coated steel sheet and the alloyed zinc coated steel sheet, corrosion-resistant steel sheets which improve their own corrosion resistance by their composition are also used. Examples of that type of corrosion-resistant steel sheet are disclosed in JP-A-54-75421 and JP-A-56-9356 (the term "P-A-" referred to herein signifies "unexamined Japanese patent publication"). Responding to the ever-increasing demand for corrosion resistance, a method to further improve the corrosion resistance of the corrosion-resistant steel sheet by applying a zinc coating to the surface of the steel sheet has also been proposed. An example of that kind of method for improving corrosion resistance is disclosed in JP-A-59-170288.
In a corrosion-resistant steel sheet, the corrosion resistance is maintained with a stable rust layer which is formed on the surface of the steel sheet during the corrosion process. To form the stable rust layer, various kinds of additives are required, and a significant amount of those additives are needed to obtain a satisfactory corrosion resistance. The additives effectively used to form the stable rust layer are, however, limited to those applied during steelmaking process and existing in the surface layer among those uniformly distributed within the steel sheet. In other words, those additives which remain inside the steel sheet are useless in terms of corrosion resistance. On the other hand, the additives also play a role to control the mechanical characteristics of the steel sheet. Accordingly, a restriction of the mechanical characteristics sometimes limits the amount of the additives for improving the corrosion resistance, which suppresses the sufficient effect of corrosion resistance.
A method disclosed in JP-A-59-170288 improves the corrosion resistance by forming a zinc coating layer on a corrosion-resistant steel sheet. However, the presented zinc coating is an alloy applied by electroplating, and the method has a disadvantage of high production cost compared with hot dip galvanizing method. The hot dip galvanizing has an advantage of lower cost than the electroplating, but the former has a disadvantage of rather poor workability and poor adhesiveness of the coating layer under a certain combination of steel sheet composition and amount of additives. The reason of the disadvantage is that the workability and adhesiveness of the hot dip galvanizing largely depend on the alloy layer formed by the reaction between the hot dip galvanized layer and the steel sheet and that a part of the additives interfere with the reaction to induce irregular formation of the alloy layer.
JP-A-3-24255 and JP-A-62-139860 disclose a method for improving the adhesiveness of coating layer by applying a "precoating" on the steel sheet surface by electroplating followed by hot dip galvanizing. This method, however, aims mainly to improve the adhesiveness of the coating layer by "pre-coating", and a very little effect of corrosion resistance is expected. In other words, this method does not consider the composition, amount, and mode of the "pre-coating" necessary to improve the corrosion resistance. JP-A-57-171692 discloses a method for coating steel sheet, where a nickel layer is coated on a steel sheet, and a nickel diffusion layer is formed within the steel sheet by heating the steel sheet in a non-oxidizing atmosphere, and a Zn--Ni alloy coating is applied on the steel sheet. Although the coating layer during corrosion process or the Ni--Fe diffusion layer after disappearance of corrosion products contributes to the improvement of corrosion resistance, but the effect is not sufficient. The reason of insufficient effect is presumably that the presence of a Ni--Fe diffusion layer stabilizes the surface condition of the steel sheet to suppress the beginning of corrosion and that once the corrosion of the base iron plate starts, however, the insufficient density of the corrosion products allows an easy permeation of water or oxygen which enhances the corrosion through the corrosion products, which can not inhibit the progress of corrosion.
JP-B-61-60914 (the term "JP-B-" referred to herein signifies "examined Japanese patent publication") discloses a technology relating to a high corrosion-resistance surface treated steel sheet. In this technology, a coating of a metal selected from the group of Zn, Ni, Sn, Cr, or an alloy consisting mainly of these metals is applied to the steel sheet. The coated steel sheet is heated in a nonoxidizing atmosphere to form a metallic diffusion layer in the steel. Then, a Zn--Ni alloy coating is applied on the metal surface. The technology, however, also gives less effect for improving corrosion resistance once the corrosion of base iron plate occurs because the components such as Zn, Ni, Sn, Cr, etc. give less contribution to the densification of the corrosion products.